[BZOJ3160]万径人踪灭(FFT+Manacher)

https://www.cnblogs.com/zhoushuyu/p/8672807.html

#include<cmath>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define rep(i,l,r) for (int i=(l); i<=(r); i++)
using namespace std;

const int N=400010,mod=1e9+7;
const double pi=acos(-1.);
int n,len,ans,L,mx,id,bin[N],rev[N],f[N],p[N];
char s[N],tmp[N];

struct C{
    double x,y;
    C(double _x=0,double _y=0):x(_x),y(_y){}
    C operator +(C &b){ return C(x+b.x,y+b.y); }
    C operator -(C &b){ return C(x-b.x,y-b.y); }
    C operator *(C &b){ return C(x*b.x-y*b.y,x*b.y+b.x*y); }
}a[N];

void FFT(C a[],int f){
    for (int i=0; i<n; i++) if (i<rev[i]) swap(a[i],a[rev[i]]);
    for (int i=1; i<n; i<<=1){
        C wn=C(cos(pi/i),f*sin(pi/i));
        for (int p=i<<1,j=0; j<n; j+=p){
            C w(1,0);
            for (int k=0; k<i; k++,w=w*wn){
                C x=a[j+k],y=w*a[i+j+k]; a[j+k]=x+y; a[i+j+k]=x-y;
            }
        }
    }
    if (f==-1) for (int i=0; i<n; i++) a[i].x/=n;
}

void work(char ch){
    for (int i=0; i<n; i++) a[i]=C(s[i]==ch,0);
    FFT(a,1); for (int i=0; i<n; i++) a[i]=a[i]*a[i]; FFT(a,-1);
    rep(i,2,len<<1) f[i]+=((int)(a[i].x+0.5)+1)/2;
}

int main(){
    freopen("bzoj3160.in","r",stdin);
    freopen("bzoj3160.out","w",stdout);
    scanf("%s",s+1); len=strlen(s+1);
    bin[0]=1; rep(i,1,len) bin[i]=(bin[i-1]<<1)%mod;
    for (n=1; n<=(len<<1); n<<=1) ++L;
    for (int i=0; i<n; i++) rev[i]=(rev[i>>1]>>1)|((i&1)<<(L-1));
    work('a'); work('b');
    rep(i,2,len<<1) ans=(ans+bin[f[i]]-1)%mod;
    rep(i,1,len) tmp[i]=s[i];
    s[0]='$'; s[1]='#';
    rep(i,1,len) s[i<<1]=tmp[i],s[(i<<1)|1]='#';
    rep(i,1,len<<1){
        p[i]=(mx>i) ? min(p[2*id-i],mx-i) : 1;
        while (s[i-p[i]]==s[i+p[i]]) p[i]++;
        if (i+p[i]>mx) mx=i+p[i],id=i;
    }
    rep(i,1,len<<1) ans=(ans-p[i]/2+mod)%mod;
    printf("%d
",ans);
    return 0;
}